An optical comparator for producing mirror images of two objects to be compared is shown and described in my co-pending application Ser. No. 706,676, filed July 19, 1976 and entitled "Optical Comparator and Inspection Apparatus." This device uses an opaque projector technique to produce mirror images of the two objects, one above the other, on a back projection screen. The apparatus is primarily suited for examining reasonably large objects, such as printed circuit boards and the like, at low magnification. Due to the back projection arrangement, a slight grain effect is inherent in the screen but is not objectionable with the type of articles for which the apparatus is used.
For comparison of very small objects at high magnification a different technique is required. In the electronics industry, very large numbers of components known as hybrids are used. These are usually in the form of a small substrate, of ceramic or other such material, carrying intricate printed circuitry to which various microcircuit chips are attached by very fine wires. Other discrete components, such as resistors, capacitors and the like may also be included, but the entire assembly is very small and requires specialized production techniques. Hybrids may cost from a few dollars to several thousand dollars each and very careful inspection techniques are necessary to insure consistent quality.
Due to the large number of hybrids used, individual inspection with reference to wiring diagrams, photographs and the like can be time consuming and costly. Blink comparators have been used, but these rely on vision persistence and are very tiring to the eyes.
For large production runs of hybrids, automatic testing equipment can be used. However, this is very expensive and is limited to the specific use for which it is designed. Also, if a fault is detected, visual inspection is still required to determine the reason.